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Re: [PSUBS-MAILIST] FW: sub ops

Hi Sean,
I don't think Doug will mind me posting this, but below is his explaination.
It's an ambient with a divers hatch in the bottom.
Howdy Alan


There is a little piece of info missing. We some air into the hard tanks to
make us slightly positive and we start a slow accent. The the cabin air
starts

to vent through the divers hatch and temporarily traps a lot of air under

the



hull in the cavity for the drop weights. Without closing the bottom valve in




the hard tanks, that air starts expanding too. By the time we hit the

surface



there is a roar of air venting through the divers hatch as well as from the
hard

tanks. That caused us the rise above the normal water line and loaded us

with



kinetic energy. Add to this heavy condensation, sun light flashing through
the

conning tower portals and surface waves rocking the boat. Then as we drop
back

the air under the hull completes venting and the momentum pushes us down so
that

the air in the hard tanks starts to compress. The thing to do is to blow all




the tanks once you hit the surface or at least close the bottom valves into
the

hard tanks. We did neither, so the hard tanks continue to compress and we
continue to sink. The automatic ambient air supply valves popped open and
started compensating the hull but the rate of decent was much faster than
they

can supply air with being feed from the surface supplied air compressor. The




scuba tanks would have done better but we held them for emergency use only.
Next thing we know is we hit the bottom in 18 or so feet of water, and
minutes

later we are again ambient. We got it on video so I'll post it later, but we
----- Original Message ----- From: "Sean T. Stevenson" <cast55@telus.net> 
To: <personal_submersibles@psubs.org>
Sent: Monday, January 17, 2011 3:11 PM
Subject: Re: [PSUBS-MAILIST] FW: sub ops



What exactly happened in Doug's incident?

-Sean


On 16/01/2011 6:01 PM, Sean T. Stevenson wrote:
I get the immediate impression that fore and aft (pitch) stability analysis is probably not being given the same care and attention as the transverse (roll) case in many cases. The same considerations apply to both, and in fact an argument can be made that fore and aft stability is more critical.
In any case, small vessels are inherently prone to significant motions due to both the sea and the movement of occupants within, and accordingly, MBTs should be designed to accommodate the worst case scenario motions (i.e. all possible motions within the point-of-no-return surface stability limits) without the loss of ballast air. For this reason, I am a fan of totally enclosed MBTs. Realistically, though, as PSubbers we are already making concessions to our intended use - apart from Carsten, I don't think anyone is meeting the mininum freeboard requirements of ABS etc. Commercially, PSub size boats like the Nuytco Deepworkers must be hoisted on deck before hatches are opened. At the Vancouver conference, by applying maximum transverse thrust in alternating directions, at the natural frequency of the vessel's roll, I was able to roll the DW to a pretty good angle - and that was still on the hook! (Phil, if you're reading this, I wasn't trying to break it - honest). The point is, it wouldn't take much effort on the part of the occupants to put a surfaced PSub hatch awash, so the question becomes, when determining to what extent you comply with the commercial design guidelines, how much safety do you make inherent to the mechanical design, versus the operating procedures and simple assumption that the vessel occupants are not going to put the vessel at risk?
As an off-the-cuff suggestion, it seems logical to me that a surfaced vessel with MBTs blown should be able to accommodate its design crew complement on the weather deck without significant risk of stability failure, when operating in the design sea state. 

-Sean


On 16/01/2011 5:16 PM, Alan James wrote:

Hi all,
This whole issue of buoyancy seems to have been under rated.
There was Tao Xianglis oil barrel sub that went bow up & had to be dragged from the water. Doug's Argonaut Junior surfaced too quickly & on the backward bounce compressed his ballast air 
& sank to the bottom before he could adjust it. & now Jim's boat.
It's a bit of a wake up call in this area.
Regards Alan





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Weare, NH  03281
603-529-1100
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